1. Field of the Invention
This invention relates to a method for the preparation of tin-containing vanadium-antimony oxide catalysts useful for the catalytic ammoxidation of C.sub.3 to C.sub.5 paraffins or olefins, more specifically for the preparation of catalysts for the ammoxidation of propane or isobutane or propylene or isobutylene to its corresponding .alpha.,.beta.-unsaturated mononitrile, acrylonitrile or methacrylonitrile. In addition, the catalyst may be used in the ammoxidation of methylpyridine, m-xylene or the oxidation of o-xylene to cyanopyridine, isophthalonitrile or phthalic anhydride, respectively.
More specifically, the invention relates to the use of a SnO.sub.2.xH.sub.2 O wherein x.gtoreq.0 dispersed in a solution of a tetraalkyl ammonium hydroxide as the reagent for tin in the preparation of catalysts containing vanadium and antimony and tin in oxide form. These types of catalysts are disclosed for instance in U.S. Pat. Nos. 3,681,421, 4,788,317, 5,008,427 and in British specifications 1,336,135 and 1,336,136, published in November 1973.
Not all sources of tin are equally effective as promoters in vanadium-antimony oxide catalysts for the oxidation and ammoxidation of saturated C.sub.3 and C.sub.4 alkanes, particularly ammoxidation. In fact, U.S. Pat. No. 5,214,016 and EPO 691306-A1 teach that the source of tin promoter is critical to the performance characteristics of the finished catalyst. It is believed that the tin should be present in very finely divided form in the precursors of such catalysts in order for the tin to be fully reactive when the solid state reaction takes place upon calcination of the catalyst precursor mixture. Tin oxide sol is a suitable source in making catalysts of the present invention; see U.S. Pat. No. 5,008,427. However, ground tin oxide or tin oxide made by reacting tin metal with nitric acid are decidedly less effective sources in catalyst preparation. While current commercially available tin oxide sol is effective, it has a serious disadvantage because it is a very expensive source.